1. Field of the Invention
The present invention relates to a displaying optical system that forms an image by scanning with a light beam. More specifically, the present invention relates to an optical system suitable for, for example, an image display apparatus such as an electronic view finder and a head-mounted display which are installed on an image pickup apparatus such as a digital camera and a digital video camera.
2. Related Background Art
Conventionally, for the electronic view finder and the head-mounted display which are installed on the digital camera, a video camera, and the like, there has been proposed an image display apparatus which allows a two-dimensional display element to display an image, and which displays the image by enlargement by an eye-piece optical system.
In this type of image display device, a large number of pixels are formed on a two-dimensional display element so that an image of a higher resolution can be displayed.
For the two-dimensional display element, there are proposed a large number of kinds of technologies using a transmission-type liquid crystal element, a reflection-type liquid crystal element, an organic EL element, and the like. Among those technologies, there exists an image display device that uses the reflection-type liquid crystal element called Liquid Crystal On Silicon (LCOS). The LCOS changes a state of polarization of an incident light between an ON state and an OFF state of each pixel. The LCOS displays an image to an observer by modulating light of each pixel, by being combined with a polarization beam splitter.
For example, Japanese Patent Application Laid-Open No. 2003-161915 (paragraphs 0012 through 0019, FIG. 1, and the like) discloses an electronic view finder that uses the LCOS.
In the two-dimensional display element such as the LCOS, in order to realize a high resolution, it is necessary to increase the number of pixels formed on the two-dimensional display element. Accordingly, it is necessary to reduce the size of each pixel formed on the display element, so that the manufacture of such an electronic view finder becomes difficult and the number of defective pixels is likely to increase due to the increase in the number of pixels.
On the other hand, for a technology for displaying a high resolution image without using the two-dimensional display element, there exists a scanning type image display device that is proposed in U.S. Pat. No. 5,467,104, for example. In this technology, scanning with a light beam emitted from a light source is performed by two-dimensional deflection means (two-dimensional scanning means), in synchronization with the modulation of the light source, and thus an image is displayed.
In such a scanning type image display device, the resolution is determined on the basis of an area of a scanned surface by the two-dimensional deflection means, a scanning angle, and the modulation of the light source. Therefore, no problem arises such as an increase in defective pixels when a high resolution is effected, contrary to the case of the two-dimensional display element.
Especially, when a micro electro mechanical systems (MEMS device) that is manufactured by applying a semiconductor process technology is used as scanning means, there is an advantage such that it is possible to carry out high-speed scanning with a device with a small size.
Further, U.S. Pat. No. 5,701,132 proposes a technology of enlarging an observation pupil of an observer by disposing NA conversion means that converts an incidence NA and an exit NA, on the scanned surface on which the light beam from the light source is condensed in the scanning type image display device. With this structure, it is possible to enlarge a pupil diameter with less deterioration of the resolution.
On the other hand, for a specific optical system of the scanning type image display device, technologies are proposed in Japanese Patent Application No. 2001-004955 (paragraphs 0236 through 0241, FIG. 20, and the like) and Japanese Patent Application No. 2001-194617 (paragraphs 0067 through 0088, FIG. 2, and the like). The technologies relate to a scanning optical system having an optical surface with one decentered reflection surface of a prism member configured by a medium having a refractive index greater than 1.
In addition, Japanese Patent Application Laid-Open No. 2004-045496 (paragraphs 0008 through 0023, FIG. 1, FIG. 2, and the like) proposes a technology of an optical system suitable for a scanning unit having a gimbal structure in which scanning of a light beam from the light source is performed in a two-dimensional direction. The optical system is a scanning optical system that has a prism member having a rotational asymmetric plane. With this structure, it is possible to obtain a high quality image by correcting a scanning distortion, and further, to provide a small video display apparatus capable of displaying a video with a high brightness by disposing an eye-piece optical system having positive power in the vicinity of the scanned surface.
However, the displaying optical system in the conventional image display apparatus has a basic structure in which a transmitted intermediate imaging plane is observed, and thus the other optical systems are disposed before or after the intermediate image. Therefore, the length of optical paths is significantly long for the optical system, and the size of the image display apparatus becomes significantly large.
Further, in Japanese Patent Application Laid-Open No. 2001-004955 (paragraphs 0236 through 0241, FIG. 20, and the like) and Japanese Patent Application Laid-Open No. 2001-194617 (paragraphs 0067 through 0088, FIG. 2, and the like), an intermediate imaging plane exists within the prism, so it is difficult to enlarge the observation pupil of the observer by disposing the NA conversion means for converting the incidence NA and the exit NA on the intermediate imaging plane, unlike the case disclosed in U.S. Pat. No. 5,701,132.
In Japanese Patent Application Laid-Open No. 2004-045496 (paragraphs 0008 through 0023, FIG. 1, FIG. 2, and the like), the eye-piece optical system having positive power is disposed in the vicinity of the scanned surface. However, the observation is not carried out by enlarging the scanned surface as a virtual image, and accordingly, it is not possible to apply this structure to a small electronic view finder and the like, as it is.